The invention relates to a method for producing a nanocomposite self-assembly and the structured materials thereby produced and, more particularly, to an evaporation-induced self-assembly method for producing nanocomposite structures and the materials thereby produced.
The exceptional strength, hardness, and toughness of biological nanocomposite systems, composed of seemingly mundane materials, has fueled considerable attention from scientists of many disciplines. Natural nanocomposites, such as shell, are formed by biomineralization, a templated, self-assembly process in which pre-organized organic surfaces regulate the nucleation, growth, morphology and orientation of inorganic crystals. Efficient synthesis and processing of layered organic/inorganic nanocomposites that mimic bone and shell structures has been a goal of the materials chemist. The most highly studied material is that of abalone shell nacre which has an oriented coating composed of alternating layers of aragonite (CaCO.sub.3) and biopolymer (.about.1 vol %). The organism fabricates the layers with precise microstructure to minimize pores and other defects. As a result, the bioceramic has esthetic qualities, smooth surface finishes and is 2-times harder and 1000-times tougher than its constituent phases.
In an attempt to mimic these examples from nature, a synthetic process termed "biomimetics" has gained momentum within the scientific community. Such approaches include crystallization beneath Langmuir monolayers, crystallization on self-assembled monolayers, supramolecular self-assembly (SSA), and sequential deposition (SD). Of these only SSA and SD offer the ability to introduce the periodic microstructural and compositional changes necessary for layered nanocomposite formation. Processes utilizing SSA have provided lamellar films but these structures collapse upon surfactant removal (Ogawa, M., J. Am. Chem. Soc., 1994, 116, 7941-7942). Stable inorganic/organic nanocomposites have been prepared with SD (see, Keinfeld, E. and Ferguson, G., Science, 1994, 265, 370-373) but this process has some experimental disadvantages as it requires many repeated deposition steps to build-up a practical coating thickness.
In forming structured materials, methods have been attempted which rely on evaporation-induced self-assembly by evaporating a component of the reaction system. For example, Bruinsma et al. (U.S. Pat. No. 5,922,299, issued on Jul. 13, 1999) describes an evaporative method of making films, fibers, and powders using an alkoxide silica precursor in a few minutes or less. Bruinsma et al. evaporate an aqueous solvent to form a structured mesoporous material but it is intentionally not a dense, non-porous film. Roth (U.S. Pat. No. 5,925,330, issued on Jul. 20, 1999) describes a method of producing a structured molecular sieve material by removing a templating surfactant, again a porous material. Brinker et al. (U.S. Pat. No. 5,858,457 issued on Jan. 12, 1999; incorporated herein by reference) describe a method for preparing mesostructured films by a solvent evaporation method using only a metal oxide, aqueous solvent and surfactant with an acidic or basic catalyst where controlled mesophase structures are prepared. Lu et al. (Lu, Y., Fan, H., Stump, A., Ward, T., Rieker, T. and Brinker, C., Nature, 1999, 398, 223-226; incorporated herein by reference) show that porous, mesostructured spherical nanoparticles can be formed within several seconds by an evaporation-induced interfacial self-assembly method.
Useful would be an efficient and simple method wherein organized inorganic/organic nanocomposite materials with little porosity can be formed within a few minutes or less. Such nanocomposite materials would have organized, polymerized phases which would lead to enhanced structural stability. Sellinger et al. (Sellinger, A., Weiss, P., Nguyen, A., Lu, Y., Assink, R., Gong, W., and Brinker, C., Nature, 1998, 394, 256-260; incorporated herein by reference) describe a method of producing such nanocomposite materials by an efficient evaporation-induced, self assembly process that results in simultaneous organization of both organic and inorganic phases to form many layers of the nanocomposite material.